Comments were closed,so I don't know any way to add more info.
The same thing happens under Camino. If I'm not logged in, I'm more likely to get the proper layout(with color background so I can see the white text).
Same behavior with Opera. When not logged in, layout is fine. When logged in, background is missing.
(All this is OSX10.3.9 FWIW).
If I might suggest a more robust layout: Never use white text. Use some dark (like black) text color and a lighter page-dependent background color. That way, even if this bug persists, lack of background colors won't interfere with our ability to read the text itself.